KR100256347B1 - The manufacturing method for pipe steelsheet with excellent anti hydrogen cracking property - Google Patents

Abstract

PURPOSE: A method for manufacturing a steel plate product for pipe is provided, which not only secures high strength and toughness but also improves hydrogen induced cracking resistance by properly controlling steel constituents, particularly appropriately adding Nb, V, Cu and Ni in steel while properly spheroidizing CaS inclusion in the steel, and optimally setting the conditions for the controlled rolling. CONSTITUTION: The method comprises the processes of controlled rolling the heated steel slab to an accumulated reduction ratio of 40 to 70% at the non-recrystallization zone of 850 to 880 deg.C after heating a steel slab comprising 0.07 to 0.13 wt.% of C, 0.35 wt.% or less of Si, 0.80 to 1.20 wt.% of Mn, 0.015 to 0.060 wt.% of soluble Al, 0.020 wt.% or less of P, 0.003 wt.% or less of S, 0.020 to 0.050 wt.% of Nb, 0.030 to 0.060 wt.% of V, 0.005 to 0.015 wt.% of Ti, 0.15 to 0.33 wt.% of Cu, 0.10 to 0.20 wt.% of Ni, 70 ppm or less of N, 50 ppm or less of Ca, and a balance of Fe and other inevitable impurities, wherein a weight ratio of Ca/S is 0.5 to 1.8, and a ratio of Ti/N is 1.0 to 3.0 in the temperature range of 1150 to 1320 deg.C; water cooling the finish rolled steel slab to a temperature of 500 to 550 deg.C in a cooling rate of 8 to 13 deg.C/sec after finish rolling the controlled rolled steel slab at a temperature on the straight line of Ar3+25 deg.C; and air cooling the water cooled steel slab to an ordinary temperature, wherein the Ca is first injected during powder injection, and injected again to be refluxed after high clean molten steel is produced by the degassing treatment, and wherein Ar3(deg.C)=869-395C+24.5Si-68.0Mn-36.1Ni-20.7Cu-24.9Cr+29.5Mo.

Description

Manufacturing method of pipe steel with excellent hydrogen organic crack resistance

1 is a scanning micrograph showing the CaS inclusions contained in the present invention and the comparative material.

2 is a photograph showing the structure after the HIC test contained in the present invention and the comparative material.

The present invention relates to a method for producing a steel for pipes used for transporting crude oil and natural gas, and more particularly, to a method for producing a steel for 50 kg / mm ² line pipe with excellent tensile strength of hydrogen-organic cracks.

Recently, as the development of crude oil and natural gas in the extreme regions of the inland and the deep sea of the sea is actively promoted, the quality of the pipe material for transportation of crude oil and natural gas extracted from the oil field is required more.

Since crude oil and natural gas usually contain hydrogen sulfide to form a corrosive sour atmosphere, pipes for transporting crude oil and natural gas (hereinafter referred to simply as "line pipes"), in particular, are particularly characterized by strength, toughness, It is essential to have resistance to Hydrogen Induced Cracking (hereinafter, simply HIC). The HIC is generated in the line pipe material because the hydrogen gas in the hydrogen sulfide is separated as time passes during the transportation of crude oil or natural gas, and the corrosion proceeds as hydrogen penetrates into the pipe material. That is, as the corrosion progresses, hydrogen diffuses into the steel, accumulates in the elongated MnS inclusions inside the steel, and becomes a starting point for cracking, and propagates the cracks by the hydrogen expansion input generated by the hydrogen molecules separated into atoms. When the crack propagates, the crack propagates along the abnormal Mn and P segregation layers (pearlite band structure).

Corrosive atmosphere that causes the HIC generation may be classified into three types according to the H ₂ S partial pressure. That is, the following Table 1 the corrosive atmosphere (sour) H ₂ S partial pressure of from about 0.0035-0.1atm corrosive atmosphere (light sour) and H ₂ S partial pressure is 0.1-1atm as shown in, and the H ₂ S partial pressure less than 1atm It is generally classified as a heavy sour, which is known to require sulfide stress corrosion cracking (SSCC) in the case of a line pipe steel used in a highly corrosive atmosphere. It is known that it is difficult to develop realistically even in advanced countries because of the shortage of test facilities and lack of test facilities. In addition, line pipe materials that can be used in a weakly corrosive atmosphere are generally high strength, high toughness steels that do not require HIC assurance. These materials are already in the stage of being actively produced beyond the development stage. On the other hand, line pipe materials used in corrosive atmospheres with H ₂ S partial pressures of less than 1 atm require HIC assurance and are currently being developed in various ways.

On the other hand, when looking at the prior art for manufacturing the HIC guarantee pipe material in the H ₂ S voltage divider environment of less than 1 atm, manufacturing methods are known as follows to prevent HIC.

That is, 1) a method of suppressing crack propagation by reducing the content of Mn, P, or the like and reducing segregation through the homogenization treatment (heat treatment) during rolling,

2) Method of suppressing HIC generation by adding rare earth element (EEM) in steelmaking to spheroidizing MnS shape,

3) A method of suppressing diffusion into hydrogen, characterized by forming a stable film on the surface of steel by adding alloying elements such as Cu, Ni, Cr, and Mo,

4) forming a microstructure by accelerated cooling after rolling to suppress the propagation of cracks, and

5) There is a method of suppressing HIC generation by optimizing MnS spheroidization by mixing the non-metallic inclusion (Ca / S) mixture 2-4 times.

However, the method of (1) of the above method cannot secure the high strength required for HIC guarantee steel even when Mn is reduced by itself, and it is difficult to increase the cost due to the heat treatment during the heat treatment and to make the pipe work due to the surface hardening of the heat treatment. There is this.

In addition, in the method of (2), sufficient MnS visualization is not achieved due to the lack of technology regarding the controllability of S in the steelmaking process.

In addition, the method of (3) is the Cr and Mo components of the plateau price, and the HIC guaranteeability is difficult because the microstructure control by cooling when rolling to the quenchability increasing element during rolling is irregular as well as the cost increase. It is known as a technique performed in the US, and has a high carbon equivalent, which has disadvantages in weldability.

In addition, the method of (4) is a recently known method, and can have a great effect on high strength and high toughness, but the HIC guarantee rate is low due to the formation of the pearlite aggregate structure, so that cooling is performed separately according to facility capacity and composition of each manufacturer. Control technology is required.

Finally, in the method of (5), the weldability is lowered in the inclusions in which Ca / S is combined 2-4 times and is currently avoided.

Accordingly, the present invention has been proposed to solve the above-mentioned conventional problems, and the present invention provides adequate steel component control, in particular, appropriately adding the Nb, V, Cu and Ni in the steel while properly visualizing and controlling the CaS reinforcement in the steel. It is an object of the present invention to provide a method for producing a thick steel plate for pipes having excellent high strength and high toughness as well as excellent resistance to cracking of hydrogen-based organics by setting rolling conditions optimally.

Hereinafter, the present invention will be described.

The present invention provides a method for producing a thick plate steel for line pipe,

By weight%, C: 0.07-0.13%, Si: 0.35% or less, Mn: 0.80-1.20%, Soluble Al: 0.015-0.060%, P: 0.020% or less, S: 0.003% or less, Nb: 0.020-0.050% , V: 0.030-0.060%, Ti: 0.005-0.015%, Cu: 0.15-0.33%, Ni: 0.10-0.20%, N: 70ppm or less, Ca: 50ppm or less, remaining Fe and other unavoidable impurities, and are composed of Ca After heating a steel slab having a weight ratio of S / S of 0.5-1.8 and a Ti / N ratio of 1.0-3.0 at a temperature range of 1150-1320 ° C., a cumulative reduction of 40-70% at 850-880 ° C., which is an uncrystallized region, Control rolling, finish rolling at a temperature directly above Ar ₃ + 25 ° C., followed by water cooling to a temperature range of 500-550 ° C. at a cooling rate of 8-13 ° C./sec, followed by air cooling to room temperature. The present invention relates to a method for producing a pipe steel having excellent organic crack resistance.

Hereinafter, the steel component and the composition according to the present invention will be described in detail.

Carbon (C) is the most contributing element to improving the strength. If the addition amount is small, the second phase fraction is lowered and the strength is decreased, while the strength is proportionally increased while the impact toughness is increased and the hardness is increased. Therefore, since the HIC resistance is reduced, the carbon (C) content is preferably limited to the range of 0.07-0.13%.

Silicon (Si) partially contributes to strength improvement, but when 0.35% or more is added, weldability is degraded and an oxide film is severely formed on the surface of the steel sheet. In addition, silicon is a deoxidizer, and since its content is most effective at 0.35% or less, it is preferable to limit the content of silicon to 0.35% or less since the addition of 0.35% or more increases the silicon-based inclusions and decreases the HIC resistance. Do.

Manganese (Mn) is an element that can improve strength and toughness at the same time. As the addition amount increases, the grains of ferrite and the second phase become finer, but when manganese content is 1.20% or more, phase martensite structure is formed and impact toughness MnS inclusions are formed together with S to decrease the HIC resistance with increasing hardness, and below 0.8% reduces the hardenability of the steel, making it difficult to form bainite into the second phase structure during subsequent accelerated cooling. Since it is difficult to secure, the content of manganese is preferably limited to 0.80-1.20%.

It is an impurity that greatly impairs the impact toughness of the steel of phosphorus (P), and it is preferable to manage it as low as possible because it accumulates in the central segregation part during performance, but it is an inevitable element in the steelmaking process. It is desirable to regulate it to 0.020% or less.

Sulfur (S) is the same harmful element as phosphorus, which can cause surface cracks, internal cracks and central segregation when played, thereby greatly reducing impact toughness, while forming MnS inclusions and inhibiting HIC content. It is desirable to limit it to 0.003% or less.

The soluble Al is added as a deoxidizer during steelmaking, but if less than 0.015%, the deoxidation effect is small, so the inclusions may be increased.When the soluble Al is more than 0.06%, the impact toughness may be inhibited and the Al ₂ O ₃ may be HIC in the pH 4.8-5.4 solution. It is desirable to manage the content of soluble Al to 0.015-0.060% because it may act as a.

Niobium (Nb) is dissolved in austenite as its amount is increased, which increases the hardenability of austenite, lowers the ferrite transformation temperature, and makes the ferrite grain size fine. Nb (C, N) increases the tensile strength to produce precipitates, and in thick plate rolling, Nb (C, N) precipitates deposited at grain boundaries inhibit grain growth, improving strength and impact toughness due to the grain refinement effect. .

However, up to 0.04% of Nb, the quality of the material is improved depending on the amount added. However, when the content of Nb is more than 0.05%, the saturation phenomenon occurs. Therefore, the strength effect does not increase. Set the range of 0.020-0.050% because it inhibits.

Vanadium (V) is a V (C, N) precipitation element, and contributes to the increase in tensile strength rather than the increase in yield strength as the content thereof increases. Therefore, considering the balance of toughness in addition to the strength aspect, it is preferable to set the content of vanadium to 0.03-0.06%. When a large amount of 0.060% or more is contained, it is possible to secure the strength with the increase of the carbon equivalent (Ceq), but the base material and the welded part It greatly impairs impact toughness.

Titanium (Ti) serves to significantly improve toughness with partial increase in strength by inhibiting initial austenite grain growth by finely dispersing TiN precipitates at grain boundaries at high temperatures such as slab reheating and welding.

However, when a large amount of Ti is contained in an amount of 0.015% or more relative to the nitrogen level (70 ppm) in molten steel, oxide-based inclusions or excessively dissolved Ti form coarse precipitates, thereby inhibiting toughness and inhibiting initial austenite grain growth when contained in 0.005% or less. The role to play is insufficient. In this invention, it is more preferable to manage the ratio of Ti / N to 1.0-3.0.

Since the Cu component contained in the steel according to the present invention has less ionization tendency than Fe, it precipitates on the surface of the steel to form a protective film on the surface, and the absorption of hydrogen can be prevented by the protective film. In other words, hydrogen atoms separated from hydrogen sulfide gas contained in crude oil or natural gas become molecules, and pressure is generated, and due to this pressure, hydrogen that tries to penetrate inside the pipe cannot be penetrated by the protective film formed by the Cu component. . At this time, in order to make the penetration force of hydrogen close to zero, it is preferable to make the content of Cu 0.15-0.33%.

However, Cu has a lower melting point than other additive elements, so when it is added alone, Cu is precipitated at about 1000 ° C during cooling in melting or reheating, and star defects occur on the surface of the slab or steel sheet. In order to prevent such defects, adding 0.10-0.20% of Ni, which forms a bond with the Cu component to increase the melting point and prevents precipitation, prevents defects due to Cu precipitation and prevents defects due to Cu precipitation. You can get it. As described above, in the present invention, the HIC resistance cannot be guaranteed at pH 4.8-5.4 without the Cu and Ni components, and further, the C or Mn down addition is essential for the HIC resistance.

That is, the present invention prevents the infiltration of hydrogen separated from hydrogen sulfide gas contained in crude oil or natural gas by adding Cu and Ni components into the steel, while appropriately lowering C or Mn, which is advantageous for HIC properties, It is characterized by compensating for the strength loss and securing the strength compensation by controlling the microstructure by the subsequent control rolling and accelerated cooling.

In addition, the present invention is characterized by further suppressing the formation of MnS inclusions in steel, forming CaS inclusions which are advantageous for HIC prevention, and spheroidizing the CaS inclusions to greatly suppress HIC generation characteristics.

In view of such inclusion composition control, in the present invention, the steel content Ca content should be 50 ppm or less while the Ca / S ratio should be in the range of 0.5-1.8. However, when the Ca content is 50 ppm or more or the Ca / S ratio is 1.8 or more, the inclusion of CaS increases, which may inhibit toughness, and when the Ca / S ratio is less than 0.5, less spherical CaS inclusions are produced. It is not preferable because the HIC characteristics cannot be sufficiently exhibited.

As described above, a preferred steelmaking manufacturing method for forming spherical CaS inclusions with Ca / S in the steel in the range of 0.5-1.8 is as follows.

That is, for proper treatment of CaS inclusions, Ca powder is injected in the powder injection treatment during the first refining to suppress MnS inclusions, and the CaS inclusion formation atmosphere is formed, followed by vacuum degassing (RH treatment). After the highly clean molten steel is generated, it is preferable to add appropriate Ca powder and reflux the molten steel within a suitable time before the MnS inclusions are produced again.

Hereinafter, the conditions for the production method according to the present invention will be described in detail.

First, the steel slab formed as described above is preferably heated at a temperature range of 1150-1320 ° C. before hot rolling, because Nb is combined with C and is present as carbide (NbC) in the slab state. This is because the temperature at which NbC can be dissolved and reclaimed in Nb and V-added steels is in the above-described range. In other words, when the reheating temperature is lower than this, the effect of reusability is less effective, and thus the yield strength required in the present invention steel cannot be secured more than 49kg / mm ² or more than 57kg / mm ² . In the present invention, it is particularly preferable to maintain at least 1150 ° C. or more for 20 minutes or more, since it is easy to secure strength due to refining the nucleation of the tissue. However, at a reheating temperature of 1320 ° C or higher, the austenite particles are so coarse that some delta-ferrite is generated in the steel, which deteriorates strength and toughness.

The slab heated as described above is subjected to controlled rolling by hot rolling in the recrystallization temperature range in a conventional manner so that the cumulative reduction ratio is 40-70% in the uncrystallized temperature range. At this time, if the cumulative reduction ratio is less than 40%, the material to meet the present invention (yield strength 49kg / mm ² or more, tensile strength 57kg / mm ² or more), in particular yield strength is impossible to secure, if exceeding 70% It is not possible to secure an appropriate finish temperature for accelerated cooling. More preferably, control rolling is carried out in the temperature range of 850-880 degreeC which is 30-60 degreeC low unrecrystallization area | region. Thereafter, after the above-mentioned control rolling is completed, rolling must be finished directly on Ar ₃ + 25 ° C. If the finish rolling end temperature is higher than Ar ₃ + 25 ° C. (when finish rolling directly on Ar ₃ + 25 ° C.), the ductility of steel, toughness, strength and HIC resistance are improved, but the finish rolling end temperature is Ar _3. If the temperature is lower than + 25 ° C, it becomes two-phase rolling, resulting in the formation of stretched ferrite and pearlite, which is very disadvantageous in terms of HIC. Therefore, the finish rolling temperature is preferably managed at a temperature directly above Ar ₃ + 25 ° C. For reference, Ar ₃ temperature is defined as in the following formula (1).

Ar ₃ (° C.) = 869-395C + 24.5Si-68.0Mn-36.1Ni-20.7Cu-24.9Cr + 29.5Mo .... (1)

On the other hand, after the above-mentioned rolling, in order to dissipate the texture generated in a part of the steel in the present invention accelerated cooling immediately after the finish rolling at Ar ₃ + 25 ℃ or more, wherein the cooling conditions of 8-13 ℃ / sec It is desirable to cool to the range and end at 500-550 ° C.

If the cooling rate, which is a means for securing strength, is lower than 8 ° C./sec, the cooling end temperature cannot be adjusted to the above range, and thus, the structure of the final steel forms ferrite + pearlite, which lowers toughness and HIC resistance. When / sec is exceeded, the strength can be secured, but a large amount of martensite low-temperature structure is formed, so that the HIC resistance is greatly reduced and the shape of the steel sheet is poor due to the increase in cooling capacity.

In addition, when cooling to match the cooling rate the end temperature may be a temperature of 500-550 ℃. Therefore, when the cooling end temperature is out of the range of 500-550 ° C the same phenomenon occurs as the reason for the limitation on the cooling rate. Thereafter, the accelerated cooled thick plate is air cooled to room temperature.

The steel plate for pipes according to the present invention prepared as described above has a uniform and fine structure of bay knife with a needle-like ferrite of about 30%, yield strength of 49kg / mm ² or more, tensile strength of 57kg / mm ² or more At the same time, it is excellent in low temperature toughness and particularly excellent in resistance to cracking of hydrogen-containing hydrogen in a corrosive atmosphere (Sour environment) of H ₂ S 1.0atm or less. Therefore, these steels are well suited for materials required in LNG or corrosive environments, as well as in line pipes for transporting crude oil or natural gas such as extreme regions.

Hereinafter, the present invention will be described in detail with reference to Examples.

EXAMPLE

Invented steels and comparative steel slabs having the composition shown in Table 2 below were manufactured through a steelmaking process treatment as shown in Table 3 below. That is, the invention steel is Ca powder is added during the powder injection treatment, Ca powder is added after RH degassing treatment, and the molten steel is refluxed for 5-10 minutes, the comparative steel is not reflux treatment or RH degassing treatment It is a steel grade not administered Ca powder. The composition ratio of Ca / S of the inventive steel and the comparative steel thus treated was measured and shown in Table 3 below.

The steel slab prepared as described above was subsequently reheated under the conditions shown in Table 4, then subjected to controlled rolling, and rapidly cooled to prepare thick plates having a thickness of 11.1, 14.3, 17.5, 19.1, and 20.6 mm. .

Then, the mechanical properties, cleanliness, and HIC degree were evaluated for the manufactured invention and the comparative material, respectively, and the results are shown in Table 5 below.

The impact test of the mechanical properties was carried out at -60 ℃ with Charpy V-notch (CVN).

In addition, HIC evaluation was evaluated for the occurrence of cracks after maintaining the specimen for 96 hours in H ₂ S saturation.

In addition, CaS inclusions for the inventive material (A) and the comparative material (C) were observed at 500 and 1000 times, respectively, using a scanning electron microscope (SEM), and the results are shown in FIG.

In addition, the invention material (A) and the comparative material (C) were observed 120 and 500 times the tissue after the HIC test, respectively, and the results are shown in FIG.

As shown in Table 5, the invention material and the comparative material are all mechanical properties such as yield strength, tensile strength, elongation, etc. within the standard, but the comparative material (CE) compared to the invention material (A) (B) It can be seen that the impact toughness and the transition temperature are very low.

In particular, it can be seen that the invention material (A) (B) is very excellent in terms of cleanliness of the steel and HIC in comparison to the comparative material (C-E), which is also confirmed in FIGS.

That is, as shown in Figure 1, in the case of the invention (A) it can be seen that the CaS inclusions (represented by a, b, c) is significantly reduced compared to the comparative material (C), as shown in the second conductor In addition, even in the state of the tissue after the HIC test, the comparative material (C) was cracked, whereas in the case of the invention (A), no crack was generated even after the HIC test.

Claims (1)

In the method for producing a thick steel plate for a line pipe, in weight%, C: 0.07-0.13%, Si: 0.35% or less, Mn: 0.80-1.20%, soluble Al: 0.015-0.060%, P: 0.020% or less, S : 0.003% or less, Nb: 0.020-0.050%, V: 0.030-0.060%, Ti: 0.005-0.015%, Cu: 0.15-0.33%, Ni: 0.10-0.20%, N: 70 ppm or less, Ca: 50 ppm or less, It is composed of the remaining Fe and other unavoidable impurities, the steel slab having a Ca / S weight ratio of 0.5-1.8 and a Ti / N ratio of 1.0-3.0 is heated at a temperature range of 1150-1320 ° C., and then the unresolved region 850-880 Control rolling at a cumulative reduction ratio of 40-70% at 0 ° C., finish rolling at a temperature directly above Ar ₃ + 25 ° C., and then water-cooled to a temperature range of 500-550 ° C. at a cooling rate of 8-13 ° C./second, and then It is configured to include air-cooling up to room temperature, wherein the Ca is injected into the powder injection first, then the degassing treatment to produce a high-purity steel, the second injection is characterized in that the hydrogen-resistant cracking pipe is excellent Method of making ash. Provided that Ar ₃ (° C.) = 869-395C + 24.5Si-68.0Mn-36.1Ni-20.7Cu-24.9Cr + 29.5Mo.